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Investment Securities (Rollforward of Fair Value Measurements by Level) (Details) | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2018 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2018 USD ($) | Sep. 30, 2017 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  |  | $ 3,208,300,000 | $ 2,547,300,000 |  |
Net realized and unrealized gains (losses) |  |  | 31,300,000 | 125,600,000 |  |
Amortization/Accretion |  |  | (2,200,000) | (6,600,000) |  |
Purchases |  |  | 1,151,900,000 | 2,078,200,000 |  |
Sales |  |  | (2,024,200,000) | (2,229,900,000) |  |
Transfers in |  |  | 0 | 13,100,000 |  |
Transfers out |  |  | 0 | (13,100,000) |  |
Ending balance | $ 2,365,100,000 | $ 2,509,400,000 | 2,365,100,000 | 2,509,400,000 |  |
Net realized and unrealized investment (losses) gains | 66,100,000 | 32,500,000 | 22,900,000 | 102,500,000 |  |
Level 1 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  |  | 890,400,000 | 279,500,000 |  |
Net realized and unrealized gains (losses) |  |  | 51,900,000 | 52,400,000 |  |
Amortization/Accretion |  |  | 100,000 | 0 |  |
Purchases |  |  | 412,600,000 | 940,700,000 |  |
Sales |  |  | (457,600,000) | (539,000,000) |  |
Transfers in |  |  | 0 | 0 |  |
Transfers out |  |  | 0 | 0 |  |
Ending balance | 897,400,000 | 733,600,000 | 897,400,000 | 733,600,000 |  |
Level 2 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  |  | 2,105,400,000 | 2,093,800,000 |  |
Net realized and unrealized gains (losses) |  |  | (33,600,000) | 59,800,000 |  |
Amortization/Accretion |  |  | (2,300,000) | (6,600,000) |  |
Purchases |  |  | 673,500,000 | 1,038,500,000 |  |
Sales |  |  | (1,565,000,000) | (1,668,200,000) |  |
Transfers in |  |  | 0 | 13,100,000 |  |
Transfers out |  |  | 0 | 0 |  |
Ending balance | $ 1,178,000,000 | 1,525,200,000 | $ 1,178,000,000 | 1,525,200,000 |  |
Number of Investments | 2 |  | 2 |  |  |
Fixed maturity investments | Level 3 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  |  | $ 0 | 0 |  |
Net realized and unrealized gains (losses) |  |  | 0 | 0 |  |
Amortization/Accretion |  |  | 0 | 0 |  |
Purchases |  |  | 0 | 31,200,000 |  |
Sales |  |  | 0 | (12,500,000) |  |
Transfers in |  |  | 0 | 0 |  |
Transfers out |  |  | 0 | (13,100,000) |  |
Ending balance | $ 0 | 5,600,000 | 0 | 5,600,000 |  |
Other long-term investments | Level 3 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  |  | 87,200,000 | 91,400,000 |  |
Net realized and unrealized gains (losses) |  |  | (4,200,000) | (2,200,000) |  |
Amortization/Accretion |  |  | 0 | 0 |  |
Purchases |  |  | 55,100,000 | 2,900,000 |  |
Sales |  |  | 0 | (2,000,000) |  |
Transfers in |  |  | 0 | 0 |  |
Transfers out |  |  | 0 | 0 |  |
Ending balance | 138,100,000 | 90,100,000 | 138,100,000 | 90,100,000 |  |
Hedge and private equity funds included in other long-term investments | Level 3 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  |  | 125,300,000 | 82,600,000 |  |
Net realized and unrealized gains (losses) |  |  | 17,200,000 | 15,600,000 |  |
Amortization/Accretion |  |  | 0 | 0 |  |
Purchases |  |  | 10,700,000 | 64,900,000 |  |
Sales |  |  | (1,600,000) | (8,200,000) |  |
Transfers in |  |  | 0 | 0 |  |
Transfers out |  |  | 0 | 0 |  |
Ending balance | 151,600,000 | 154,900,000 | 151,600,000 | 154,900,000 |  |
Short-term investments |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Net realized and unrealized investment (losses) gains |  |  | (700,000.0) |  |  |
Foreign Exchange Forward |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Net realized and unrealized investment (losses) gains |  |  | (3,500,000.0) | (23,100,000.0) |  |
Foreign Currency on Cash and Open Trades |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Net realized and unrealized investment (losses) gains |  |  | (4,200,000.0) |  |  |
Fair value measured on a recurring basis |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  |  | 3,259,100,000 |  |  |
Ending balance | 2,527,200,000 |  | 2,527,200,000 |  |  |
Fair value measured on a recurring basis | Level 1 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  |  | 1,041,400,000 |  |  |
Ending balance | 1,198,100,000 |  | 1,198,100,000 |  |  |
Fair value measured on a recurring basis | Level 2 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  |  | 2,130,500,000 |  |  |
Ending balance | 1,191,000,000 |  | 1,191,000,000 |  |  |
Fair value measured on a recurring basis | Level 3 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  |  | 87,200,000 |  |  |
Ending balance | 138,100,000 |  | 138,100,000 |  |  |
Fair value measured on a recurring basis | Fixed maturity investments |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  |  | 2,129,700,000 |  |  |
Ending balance | 1,031,900,000 |  | 1,031,900,000 |  |  |
Fair value measured on a recurring basis | Fixed maturity investments | Level 1 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  |  | 296,500,000 |  |  |
Ending balance | 125,400,000 |  | 125,400,000 |  |  |
Fair value measured on a recurring basis | Fixed maturity investments | Level 2 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  |  | 1,833,200,000 |  |  |
Ending balance | 906,500,000 |  | 906,500,000 |  |  |
Fair value measured on a recurring basis | Fixed maturity investments | Level 3 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  |  | 0 |  |  |
Ending balance | 0 |  | 0 |  |  |
Fair value measured on a recurring basis | Other long-term investments |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  |  | 87,200,000 |  |  |
Ending balance | 138,100,000 |  | 138,100,000 |  |  |
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  |  | 0 |  |  |
Ending balance | 0 |  | 0 |  |  |
Fair value measured on a recurring basis | Other long-term investments | Level 2 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  |  | 0 |  |  |
Ending balance | 0 |  | 0 |  |  |
Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  |  | 87,200,000 |  |  |
Ending balance | 138,100,000 |  | 138,100,000 |  |  |
Fair value measured on a recurring basis | Short-term investments |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  |  | 176,100,000 |  |  |
Short-term investments, at fair value | 313,700,000 |  | 313,700,000 |  |  |
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  |  | 151,000,000 |  |  |
Ending balance | 300,700,000 |  | 300,700,000 |  |  |
Short-term investments, at fair value |  | 786,500,000 |  | 786,500,000 | $ 176,100,000 | $ 175,000,000
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  |  | 25,100,000 |  |  |
Ending balance | 13,000,000 |  | 13,000,000 |  |  |
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Beginning balance |  |  | 0 |  |  |
Ending balance | $ 0 |  | $ 0 |  |  |
Forward Contracts | Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Derivative, fair value, net |  | $ (15,400,000) |  | (15,400,000) | $ (3,700,000) | (1,200,000)
Star & Shield Insurance Exchange |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Deconsolidation of SSIE |  |  |  | 5,200,000 |  |
Star & Shield Insurance Exchange | Level 1 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Deconsolidation of SSIE |  |  |  | 0 |  |
Star & Shield Insurance Exchange | Level 2 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Deconsolidation of SSIE |  |  |  | 5,200,000 |  |
Star & Shield Insurance Exchange | Fixed maturity investments | Level 3 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Deconsolidation of SSIE |  |  |  | 0 |  |
Star & Shield Insurance Exchange | Other long-term investments | Level 3 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Deconsolidation of SSIE |  |  |  | 0 |  |
Star & Shield Insurance Exchange | Hedge and private equity funds included in other long-term investments | Level 3 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Deconsolidation of SSIE |  |  |  | $ 0 |  |
Star & Shield Insurance Exchange |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Trading securities debt reclassified to assets held-for-sale |  |  |  |  |  | 6,600,000
Star & Shield Insurance Exchange | Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Short-term investments, at fair value |  |  |  |  |  | $ 100,000

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